The present invention relates to a process control instrument. In particular, it relates to a pressure transmitter which couples to a process pressure.
Transmitters which sense pressure have a pressure sensor typically coupled to at least one isolation diaphragm. The transmitters typically attach to a flange and include an opening which is aligned to receive process fluid from a passageway in the flange. The isolation diaphragm is positioned in the opening, and isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor having a sensing diaphragm through a substantially incompressible isolation fluid carried in a passageway. U.S. Pat. No. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and U.S. Pat. No. 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION show pressure transmitters of this type.
Pressure transmitters frequently include a weld ring positioned within the opening surrounding the isolation diaphragm. The weld ring can be welded to the transmitter body and to the isolation diaphragm, thus securing the isolation diaphragm to the transmitter body. The weld ring can also be used to support a seal, such as an O-ring. When the transmitter is attached to the process flange by an attachment force, such as provided by bolts, the seal and weld ring are compressed against the flange to prevent process fluid from leaking past the opening. Examples of such weld rings are shown in U.S. Pat. No. 5,922,965, to Behm et al., entitled PRESSURE SENSOR AND TRANSMITTER HAVING A WELD RING WITH A ROLLING HINGE POINT, granted Jul. 13, 1999 and U.S. Pat. No. 6,055,863, to Behm et al., entitled PRESSURE SENSOR AND TRANSMITTER HAVING A WELD RING WITH A ROLLING HINGE POINT, granted May 2, 2000 and both assigned to Rosemount Inc.
Some types of seals used in connection with the weld rings are particularly difficult to maintain. For example, a seal comprising Polytetrafluoroethylene (PTFE) can be used as a seal. However, PFTE has a tendency to “cold flow” and lose its shape over time. This can reduce the pre-load force applied to the seal and thereby reduce the effectiveness of the seal.